Ostracods (PA-17) Ostracods are tiny bivalved crustaceans that
live in both fresh and salt water but are most abundant in the seas. They
range from less than 1 mm to more than 20 mm in length. Most move in vast
swarms, swimming near the surface or crawling over the shallow sea bottom
where they pursue their scavenging habits.

The ostracod body is
contained within a horny or calcerous shell, to which it is attached by
muscles. The muscles close the hinged valves and operate several pairs of
appendages that are used for sensors, locomotion, eating and respiration.
Like all arthropods, ostracods molt the shell for growth; some do so eight
times before they reach their final size and shape.

Ostracods appeared
in the seas more than 500 million years ago and have been abundant ever
since. They are common in Indiana rocks. Some have smooth shells, but
others are marked by ridges, knobs, pits and furrows, all of which are
used for identification by paleontolgists. Because many species spread
widely but did not live very long, they make excellent index fossils. They
are also useful for understanding ancient environments.